Georgia Southern University Digital Commons@Georgia Southern University Honors Program Theses 2018 Quantum Chemical Analysis of Stable Noble Gas Cations for Astrochemical Detection Carlie M. Novak Georgia Southern University Follow this and additional works at: https://digitalcommons.georgiasouthern.edu/honors-theses Part of the Physical Chemistry Commons, and the Theory and Algorithms Commons Recommended Citation Novak, Carlie M., "Quantum Chemical Analysis of Stable Noble Gas Cations for Astrochemical Detection" (2018). University Honors Program Theses. 311. https://digitalcommons.georgiasouthern.edu/honors-theses/311 This thesis (open access) is brought to you for free and open access by Digital Commons@Georgia Southern. It has been accepted for inclusion in University Honors Program Theses by an authorized administrator of Digital Commons@Georgia Southern. For more information, please contact [email protected]. Quantum Chemical Analysis of Stable Noble Gas Cations for Astrochemical Detection An Honors Thesis submitted in partial fulfillment of the requirements for Honors in the Department of Chemistry and Biochemistry By Carlie M. Novak Under the mentorship of Dr. Ryan C. Fortenberry ABSTRACT The search for possible, natural, noble gas molecules has led to quantum chemical, spec- troscopic analysis of NeCCH+, ArNH+, ArCCH+, and ArCN+. Each of these systems have been previously shown to be a stable minimum on its respective potential energy surface. However, no spectroscopic data are available for laboratory detection or interstellar obser- vation of these species, and the interstellar medium may be the most likely place, in nature, where these noble gas cations are found. The bent shape of NeCCH+ is confirmed here with a fairly large dipole moment and a bright C { H stretching frequency at 3101.9 cm−1. Even if this molecule is somewhat unstable, it is likely observable now that the spectral ranges of where to look have been established. ArCCH+ is much more stable but has dim dou- ble harmonic intensities for the vibrational fundamentals and a dipole moment below 0.5 D making its rotational transitions likely buried in the astronomical weeds. Even so, ArCCH+ cannot be excluded as a possibility in laboratory experiments of hydrocarbons in argon-rich environments. ArCN+, on the other hand, has a dipole moment of greater than 3.5 D, an observable C { N stretching fundamental at 2189.6 cm−1 (4.567 microns), and a viable formation pathway through HCN, a highly-abundant interstellar molecule. Consequently, the result of this work provides novel high-level spectroscopic data to aid in the search for covalently bound noble gas molecules in observable regions of outer space. Thesis Mentor: Honors Director: Contents 1 Introduction 1 1.1 Astrochemistry . 1 1.1.1 Noble Gas Chemistry . 2 1.2 Theoretical Chemistry . 3 1.2.1 Quantum Chemical Techniques . 4 1.2.2 Coupled Cluster Theory . 5 1.2.3 Quartic Force Fields . 7 2 Covalency in a Noble Gas Compound: ArNH+ Analyzed from Quartic Force Fields 9 2.1 Introduction . 9 2.2 Computational Details . 12 ii 2.3 Results and Discussion . 13 2.4 Conclusions . 17 3 The Rovibrational Spectra of Three, Stable Noble Gas Molecules: NeCCH+, ArCCH+, and ArCN+ 19 3.1 Introduction . 19 3.2 Computational Details . 23 3.3 Results and Discussion . 26 3.3.1 NeCCH+ .................................. 26 3.3.2 ArCCH+ .................................. 28 3.3.3 ArCN+ ................................... 30 3.4 Conclusions . 32 4 Conclusions 38 Bibliography 42 iii List of Tables 2.1 The ArNH+ CcCR Simple-Internal Force Constants (in mdyn/A˚n·radm). 17 + 2.2 The CcCR QFF ArNH Equilibrium and Zero-Point (Rα) Structures, Spec- troscopic Constants, Vibrational Frequencies, and Harmonic Intensities (in parenthesis in km/mol) for the 36Ar, 38Ar, 40Ar, H, and D-Including Isotopo- logues. 18 3.1 The NeCCH+ CcCR Internal Force Constants (in mdyn/A˚n·radm). 29 + 3.2 The NeCCH CcCR QFF Equilibrium and Zero-Point (Rα) Structures, Spec- troscopic Constants, Vibrational Frequencies, and Harmonic Intensities (in parenthesis in km/mol) for the 20Ne, 22Ne, H, and D Isotopologues. 34 3.3 The ArCCH+ CcCR Symmetry-Internal Force Constants (in mdyn/A˚n·radm). 35 iv + 3.4 The CcCR QFF ArCCH Equilibrium and Zero-Point (Rα) Structures, Spec- troscopic Constants, Vibrational Frequencies, and Harmonic Intensities (in parenthesis in km/mol) for the 36Ar, 38Ar, 40Ar, H, and D-Including Isotopo- logues. 36 3.5 The ArCN+ CcCR Symmetry-Internal Force Constants (in mdyn/A˚n·radm). 36 + 3.6 The CcCR QFF ArCN Equilibrium and Zero-Point (Rα) Structures, Spec- troscopic Constants, Vibrational Frequencies, and Harmonic Intensities (in parenthesis in km/mol) for the 36Ar, 38Ar, 40Ar . 37 v Chapter 1 Introduction 1.1 Astrochemistry Astrochemistry is defined as the study of molecules and their subsequent reactions in environ- ments beyond the Earth. Environments of interest include solar system bodies, exoplanets along with their atmospheres, and the interstellar medium (ISM) among many, many others. Through this discipline, the formation, atomic and chemical composition, and evolution of molecular gas clouds can be studied in order to predict or describe how solar systems are formed. Spectroscopy is a particularly important tool in astrochemistry because we can not collect samples or study the molecules directly. Astrochemical spectroscopy uses various telescopes to measure the absorption and emission of light form molecules and atoms in dif- ferent interstellar environments. By comparing astronomical observations with experimental 1 Carlie M. Novak Chapter 1. Introduction 2 measurements, chemists can infer numerous physical and chemical qualities of stars and gas clouds based on their rotational and vibrational characteristics. The astrochemist's periodic table consists of only 11 elements50, whereas the other 81 are more commonly associated with terrestrial chemistry. These elements are largely inaccessible in the ISM as their abundances are too low to be considered for most astrochemistry. Three of these 11 elements are the noble gases: helium, neon, and argon. Noble gases species are popularly assumed not to partake in chemical reactions, as they are full valence atoms with high ionization energies that are stable on their own. Through the use of quantum chemical analysis, theoretical astrochemistry allows us to challenge standard chemical assumptions, and discover new and bizarre molecules that break the rules of traditional "terrestrial" chemistry. 1.1.1 Noble Gas Chemistry Proceeding its theoretical characterization in the early 1990s, argonium, ArH+, was detected in the Crab nebula in 201325. Despite the previous knowledge that noble gases do not possess the reactivity necessary to form bonds, work in recent years has cast doubt on this idea1{7. It was discovered that the reason it took so long to find the ArH+ molecule was because astronomers were looking for argon in its most experimentally available isotope, 40Ar. However, this is only true of the atom within the confines of Earth's atmosphere. In the ISM, argon is most abundant in its isotope, 36Ar. Now that the correct isotope, specifically Carlie M. Novak Chapter 1. Introduction 3 for argon,is known, more accurate predictions regarding novel noble gas molecules can be made. In order to expand the list of potential noble gas molecules, highly-accurate QFFs methods are employed here to analyze unique noble gas radical cations for the first time. Recent + + work has explored and produced quantum chemical spectroscopic data for ArH2 , ArH3 , ArOH+, and their neon analogues using established QFF techniques4,8,9. 1.2 Theoretical Chemistry Theoretical chemistry is extremely useful for rotational astrochemistry because the low pres- sures, densities, and temperatures of the ISM are better at creating nearly perfectly isolated molecules than any experiments in terrestrial laboratories. Quantum chemical computations of lone molecules can be physically interpreted to be infinitely separated from any other molecular species and are treated without consideration for temperature since such would not make sense for a lone molecule. Additionally, modern quantum chemical computations, especially coupled cluster theory at the singles, doubles, and perturbative triples [CCSD(T)] level are exceptionally adept at providing molecular structures10. From such structures, related rotational constants are easily computed, providing reference for rotational spec- tra. Dipole moments are also readily determined form these structures and from electron probability computations allowing for inferences related to the intensities of the rotational lines. Carlie M. Novak Chapter 1. Introduction 4 1.2.1 Quantum Chemical Techniques The basis of quantum chemistry is to solve the Shr¨odingerequation, a typical Eigenvalue equation (Eq. 1.1) specifically contructed to deal with molecular wavefunctions and Hamil- tonians. The Hamiltonian, (Eq. 1.2), contains the operations associated with kinetic and potential energies of the entire molecular system. A majority of quantum chemical studies use the Born-Oppenheimer (BO) Approximation, which assumes that the nucleus is at rest ^ ^ and therefore TN = 0. This also assumes that the VNN terms is a constant since the nuclei are implied not to be moving. The Hamiltonian is then reduced to include its electronic components and is shown in Eq. 1.3. H^ Ψ = EΨ (1.1) ^ ^ ^ ^ ^ ^ H = Te + TN + VeN + Vee + VNN (1.2) ^ ^ ^ ^ HBO = Te + VeN + Vee (1.3) The Hartree-Fock (HF) method